1. Field of the Invention
This invention relates to a universal joint having a rotation body and a rotation shaft which cause bending in any direction.
2. Description of the Related Art
FIG. 4 to FIG. 8 show an example conventionally known as the above type of universal joints. The conventional universal joint in FIGS. 4 to 8 has ball portions 2 and 3 respectively provided at the ends of a rotation shaft 1. The ball portions 2 and 3 have the respective pairs of projections 4 and 5 formed on the diameter lines of the ball portions 2 and 3.
Two rotation bodies 6 and 7, which are coupled to the rotation shaft 1, are formed in a tubular shape and respectively have holding holes 8 and 9 formed on their axis. The holding holes 8 and 9 respectively receive the ball portions 2 and 3 and allow them to slide in the axis direction. The rotation body 6 has guide grooves 10 formed in positions 180 degrees out of phase with each other, and the rotation body 7 has guide grooves 11 formed in a like manner. The guide grooves 10 slidably receive the respective projections 4 of the ball portion 2 which is placed in the holding hole 8, and likewise the guide grooves 11 slidably receive the respective projections 5.
By placing the ball portions 2 and 3 in the respective holding holes 8 and 9 in this manner, the ball portions 2 and 3 can rotate about the axis line of the projections 4 and 5, and further rotate while tilting the projections 4 and 5 in the guide grooves 10 and 11. Thus the rotation shaft 1 is capable of rotating in any direction with respect to the rotation bodies 6 and 7.
For example, a universal joint structured as described above is used to couple a drive wheel of a radio-controlled car or the like to an output shaft or the like. When a drive wheel of a radio-controlled car is coupled to an output shaft, the rotation body 6 is coupled to the drive wheel as shown in FIG. 6. Specifically, the rotation body 6 is mounted, via bearings 14 and 15, in a ring portion 13 formed in a steering knuckle arm 12 for the drive wheel. The drive wheel 17 is fixed to an axle 16 provided integrally with the rotation body 6. Accordingly, upon the rotation of the rotation shaft 1, the rotation body 6 rotates and the drive wheel 17 rotates with the rotation of the rotation body 6. The knuckle arm 12 moves rotationally in directions of the arrows 18 shown in FIG. 6, and the center of the rotational motion is a point X corresponding to the center of a kingpin (not shown).
The other rotation body 7, which is located opposite to the rotation body 6, is coupled to an output shaft (not shown) and rotates in conjunction with the output shaft. Therefore, the torque of the rotation body 7 rotating along with the output shaft is transmitted through the rotation shaft 1 to the rotation body 6. After the torque is transmitted to the rotation body 6 in this manner, the torque is transferred also to the drive wheel 17 to produce rotation of the drive wheel 17. At this point, if the knuckle arm 12 moves rotationally in either of the directions of the arrows 18, the drive wheel 17 is changed in direction.
When the knuckle arm 12 moves rotationally in either of the directions of the arrows 18 about the X point where the kingpin is provided as described above, a virtual distance between the opposing rotation bodies 6 and 7 increases. For example, when the rotation bodies 6 and 7 are on the same axis, the distance between the rotation bodies 6 and 7 is L1 as illustrated in FIG. 7. When the rotation bodies 6 and 7 are placed on the same axis as described above, the rotation center X of the rotation body 6 and the rotation center Y of the ball portion 2 are considered to align with each other.
If the rotation body 6 rotates from the above position about the rotation center X, the distance between the rotation bodies 6 and 7 becomes L2 as shown in FIG. 8. The distance L2 becomes longer than the distance L1. However, even if the opposite distance between them increases from L1 to L2 as described above, the rotation shaft 1 is not extendable, and therefore a difference between the two distances is absorbed by moving the ball portions 2 and 3 in the holding holes 8 and 9 of the rotation bodies 6 and 7 in the axial direction.
A specific examination is not made for the conventional example.
Regarding conventional universal joints structured as described above, for example, when the rotation body 6 rotates about the position X, the distance between the rotation bodies 6 and 7 is increased by a length “L2−L1”. For example, it is assumed that the increased length is absorbed in the rotation body 6 of the two rotation bodies. If the increased distance is absorbed in the rotation body 6 in this manner, the rotation center Y of the ball portion 2 becomes out of alignment with the rotation center X of the rotation body 6 as shown in FIG. 8.
The misalignment caused between the rotation center X of the rotation body 6 and the rotation center Y of the ball portion 2 as described above makes it impossible for the rotation body 6 to rotate about the center X. Still, the knuckle arm 12 continues to rotate about the center X in order to change the direction the drive wheel 17 travels. However, at this point, the rotation body 6 rotates while pulling the rotation center Y of the ball portion 2 to the rotation center X. In other words, the rotation body 6 rotates while moving the rotation shaft 1 toward the rotation body 6 in such a manner as to draw the ball portion 3 located at the other end of the rotation shaft 1 out from the holding hole 9 of the rotation body 7.
If the rotation body 6 rotates while pulling the ball portion 2 inward as described above, the resistance is increased, thereby making it difficult to smoothly change the direction the drive wheel 17 travels. Further, every time the direction the drive wheel 17 travels is changed as described above, the rotation shaft 1 moves in the axial direction. Thus, a smooth change in the direction the drive wheel 17 travels is made difficult.
Note that the foregoing description is given of an example of integrating the drive mechanism of the drive wheel 17 and the steering mechanism with each other and systematically rotating the rotation body 6 of the two rotation bodies about the center X. However, in the aforementioned conventional universal joints, the rotation of the rotation body is not caused systematically, but is caused as a consequence. In this case, problems as described in the forgoing also arise.
Any problem as described in the forgoing does not arise if the rotation center Y of the ball portion 2 in the holding hole 8 and the rotation center X of the rotation body 6 are in alignment with each other at all times. However, in view of the cost merits, the conventional universal joint as described above is not designed such that the rotation center Y of the ball portion 2 in the holding hole 8 and the rotation center X of the rotation body 6 are systematically aligned with each other. In order to achieve the positional alignment between the rotational center Y of the ball portion 2 in the holding hole 8 and the rotation center X of the rotation body 6, this type of inexpensive universal joint is considered incompetent, and the use of a higher precision universal joint is considered necessary.
Further, the holding hole 8 needs to be deepened for ensuring the amount of movement of the center Y of the ball portion 2. However, when the ball portion 2 is positioned close to the closed end of the holding hole 8, if the rotation shaft 1 moves toward the rotation body 6, disadvantageously, the rotation shaft 1 is pressed against the opening edge of the holding hole 8 and causes damage to the opening edge.